1. Field of the Invention
The present invention relates to a boron nitride complex and a process for its preparation and a process for preparing a light-transmitting dense body of cubic system boron nitride. In the present invention, the boron nitride complex is meant for boron nitride in which lithium or an alkaline earth metal is diffusely supported in the form of its boron nitride.
2. Description of the Prior Art
When a catalyst is used for a chemical reaction, it is common that a catalytically active substance is fixed or supported on a solid carrier to enlarge the effective surface area of the catalytically active substance, to ensure the uniform reaction, to prevent the catalyst from flowing away or to increase the recovery rate of the catalyst. A number of such catalyst systems have been known so far.
Hexagonal system boron nitride (hereinafter referred to as "hBN") is a chemically stable solid having a layered structure similar to the structure of graphite. However, it has not been known to employ such boron nitride as a carrier for supporting lithium or an alkaline earth metal.
Heretofore, as a process for the synthesis of cubic system boron nitride (hereinafter referred to as "cBN"), it has been known to mix hBN with an alkali metal or alkaline earth metal nitride or boron nitride and treat the mixture under a high temperature and pressure.
Further, the present inventors have previously developed a process for the production of a sintered body of cBN which comprises mixing hBN with magnesium boron nitride, and sintering the mixture under a high temperature and pressure (Japanese Patent Application No. 57549/1982).
In these conventional processes, the mixing with the alkali metal or alkaline earth metal boron nitride is conducted by a mechanical means. By such mechanical mixing, it is difficult to uniformly distribute the alkali metal or alkaline earth metal boron nitride with high dispersibility to hBN. Besides, the alkali metal or alkaline earth metal boron nitride (except for magnesium boron nitride) is highly reactive with moisture, and accordingly, if the mixing operation is conducted for an extended duration to ensure uniform mixing, it is likely to react with moisture and lose its catalytic activity and thus tends to degrade the quality of the product thereby obtained. Therefore, it is thereby impossible to obtain cBN having a high quality. Although it is possble to obtain a light-transmitting sintered product of cBN, the product usually has cloud-like or spot-like haze and a yellow color and it is unsatisfactory in its quality.
Cubic system boron nitride (cBN) having a zincblende structure has hardness comparable to diamond, and is thermally and chemically more stable than diamond. By virtue of these superior properties and reflecting the recent trend for automation or saving of man power for the operation of machining tools by means of computer control and the like, cBN has been recognized as a prospective material suitable for highly efficient and precise tools. Particularly, cBN is known to provide excellent performance in the processing of iron-family materials such as high speed steel, die steel, cast iron and sintered hard alloys. Further, cBN has promising properties for use as electronic materials and optical materials. Therefore, it is desired to develop the technologies for the production of a sintered body of cBN having a high purity and density, as well as its single crystal.
For the production of a sintered product of cBN, the following methods have been known:
(1) A method in which hBN having various characteristics such as the particle size, crystallinity and the structural specificity is used as the starting material, and it is treated under a high temperature and high pressure condition. (e.g. Japanese Examined Patent Publication No. 17520/1977)
(2) A method in which hBN is activated by pretreatment such as reduced pressure treatment, and the activated hBN is used as the starting material. (Japanese Unexamined Patent Publication No. 167110/1980)
However, the sintered products of cBN obtainable by these methods are grayishly translucent or blackishly opaque and cannot yet be recognized as a highly hard dense body wherein cBN particles are directly bonded to one another.
The present inventors have earlier developed a process for producing a light-transmitting dense product of cBN which comprises mechanically mixing hBN or a mixture of hBN and cBN with from 0.15 to 3.0 molar % of Me.sub.3 B.sub.2 N.sub.4 where Me is an alkaline earth metal, and sintering the mixture thus obtained as a starting material under high pressure. (Japanese Patent Application No. 57549/1982)
As mentioned above, however, the sintered product of cBN obtainable from a mechanically mixed starting material is not totally satisfactory in its quality.